Effect of Congo Red on Swarming and Cell Morphology. A. P. vortex streaked on MH agar (1.5 % w/v) with 400 μg/ml Congo Red and 100 μg/ml X-gal (to add contrast for photography). B. SEM of cells cultured with Congo 400 μg/ml Red with frequent convoluted/bent cells and absence of flagella (compare with Fig 3). C. Spot plating on Congo Red showing ringed colony but no motility after 48 h. 24 h point the cells in the control plates without Congo Red had reached the edges. Scale bar in panel (B) is 2 cm when applied to (A), 1 μm for (B) and 1 cm for (C).